Long life mold tool steel with improved physical properties at high temperature and mold using the same

ABSTRACT

The present invention relates to an alloy steel for a mold tool and a mold using the same. 
     The steel for the mold tool comprises iron (Fe) as a main component, an amount of about 0.35 to 0.45 wt % of carbon (C), amount of about 0.80 to 1.20 wt % of silicon (Si), amount of about 0.20 to 0.50 wt % of manganese (Mn), amount of about 6.00 to 8.00 wt % of chromium (Cr), amount of about 1.50 to 3.00 wt % of molybdenum (Mo), and amount of about 0.80 to 1.20 wt % of vanadium (V) based on the total weight of the tool steel. Accordingly, the mold tool have improved physical properties at high temperature and extended life span.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Korean PatentApplication No. 10-2016-0027705, filed on Mar. 8, 2016, in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to an alloy steel for a mold tool that hasimproved physical properties at high temperature and extended life span,a mold using the alloy steel. In particular, the alloy steel maycomprise molybdenum, tungsten, and niobium.

BACKGROUND OF THE INVENTION

Recently, as global environmental problems have emerged, the entireindustry has searched for methods of reducing fuel in order to cope withthese problems. Among them, a vehicle industry has been developingvarious environmentally friendly vehicles aiming at reducing carbondioxide emissions to about 95 g/km which is a about 27% level comparedto the current level by 2021 in accordance with the European regulation.Further, the vehicle makers have strived to develop technologies ofdownsizing and enhancing fuel efficiency in order to meet the standard,for example, 54.5 mpg (23.2 km/l), which is set by the US CorporateAverage Fuel Economy (CAFÉ) before 2025.

In order to achieve the reduction of fuel consumption, two methods suchas improvement in the efficiency of a vehicle engine and reduction inthe weight of a vehicle have been suggested a solution by the vehicleindustry sector. The reduction in the weight of a vehicle may enhancethe fuel efficiency of the vehicle. However, when the weight of thevehicle is reduced, the strength of alloy steel applied to the vehiclemay be improved, and as a result, the lifespan of a mold whichmanufactures the vehicle may be reduced. Accordingly, the vehicleindustry has had a goal to solve that problem.

When this matter is further reviewed, recently, there have occurredproblems of an increase in load imposed on the mold according to anincrease in strength of parts for reducing the weight of the vehicle andimprovement in productivity in the vehicle industry. Recently, thenumber of parts produced from one mold has been increased in order toincrease the productivity in the production of products by the mold. Forexample, FIG. 1 shows a change in cavity of a mold according to therelated art. As illustrated in FIG. 1, unlike the production of one partfrom a mold including one cavity-mold 1, that is, a mold including onecavity, two cavity-mold 3, that is, two parts including two cavities inone mold have been recently produced. Further, the strength of parts mayincrease due to the reduction in weight of a vehicle. FIG. 2 showsbreakage of a forging mold according to the related art. As illustratedin FIG. 2 as the R engine con rod may be applied as a high strengthmaterial, the strength may be increased to about 50 K compared to atypical strength of 35 K in the related art. The load imposed on themold may be increased when an R engine con rod is manufactured, and thusthe mold may be damaged. Due to the fact stated above, the improvementin physical properties of a mold material at high temperature caused byan increase in forging load has recently emerged as a main issue in thevehicle industry.

SUMMARY OF THE INVENTION

In preferred aspects, the present invention provides an alloy steel withimproved physical properties at high temperature and an extended life,and a mold using the same.

Further, the alloy steel, i.e. alloy steel composition, may comprisemolybdenum, tungsten, and/or niobium to provide improved strength athigh temperature, enhanced temper softening resistance, enhanced hotabrasion resistance, and improved toughness.

The technical problems which the present invention intends to solve arenot limited to the technical problems which have been mentioned above,and still other technical problems which have not been mentioned will beapparently understood to a person with ordinary skill in the art fromthe description of the present invention.

An exemplary embodiment of the present invention provides an alloy steelthat may comprise an amount of about 0.35 to 0.45 wt % of carbon (C), anamount of about 0.80 to 1.20 wt % of silicon (Si), an amount of about0.20 to 0.50 wt % of manganese (Mn), an amount of about 6.00 to 8.00 wt% of chromium (Cr), an amount of about 1.50 to 3.00 wt % of molybdenum(Mo), an amount of about 0.80 to 1.20 wt % of vanadium (V), and iron(Fe) constituting the remaining balance of the alloy steel, based on thetotal weight of the alloy steel.

Unless otherwise indicated, the wt % of the components is based on thetotal weight of the alloy steel.

Preferably, the alloy steel may further include niobium (Nb). A contentof the niobium (Nb) suitably may be an amount of about 0.05 to 0.10 wt %based on the total weight of the alloy steel.

Preferably, the alloy steel may further include tungsten (W). A contentof the tungsten (W) suitably may be an amount of about 0.10 to 1.00 wt %based on the total weight of the alloy steel.

Preferably, the alloy steel may further include niobium (Nb) andtungsten (W). A content of the niobium (Nb) suitably may be an amount ofabout 0.05 to 0.10 wt % and a content of the tungsten (W) suitably maybe an amount of about 0.10 to 1.00 wt % based on the total weight of thealloy steel.

Further provided are the alloy steels that may consist essentially of,essentially consist of, or consist of the components as described above.For example, the alloy steel composition may consist essentially of,essentially consist of, or consist of: an amount of about 0.35 to 0.45wt % of carbon (C), an amount of about 0.80 to 1.20 wt % of silicon(Si), an amount of about 0.20 to 0.50 wt % of manganese (Mn), an amountof about 6.00 to 8.00 wt % of chromium (Cr), an amount of about 1.50 to3.00 wt % of molybdenum (Mo), an amount of about 0.80 to 1.20 wt % ofvanadium (V), and iron (Fe) constituting the remaining balance of thealloy steel, based on the total weight of the alloy steel.

The alloy steel may also consist essentially of, essentially consist of,or consist of: an amount of about 0.35 to 0.45 wt % of carbon (C), anamount of about 0.80 to 1.20 wt % of silicon (Si), an amount of about0.20 to 0.50 wt % of manganese (Mn), an amount of about 6.00 to 8.00 wt% of chromium (Cr), an amount of about 1.50 to 3.00 wt % of molybdenum(Mo), an amount of about 0.80 to 1.20 wt % of vanadium (V), an amount ofabout 0.05 to 0.10 wt % niobium (Nb), and iron (Fe) constituting theremaining balance of the alloy steel, based on the total weight of thealloy steel.

Further, the alloy steel may consist essentially of, essentially consistof, or consist of: an amount of about 0.35 to 0.45 wt % of carbon (C),an amount of about 0.80 to 1.20 wt % of silicon (Si), an amount of about0.20 to 0.50 wt % of manganese (Mn), an amount of about 6.00 to 8.00 wt% of chromium (Cr), an amount of about 1.50 to 3.00 wt % of molybdenum(Mo), an amount of about 0.80 to 1.20 wt % of vanadium (V), an amount ofabout 0.10 to 1.00 wt % of tungsten (W), and iron (Fe) constituting theremaining balance of the alloy steel, based on the total weight of thealloy steel.

Moreover, the alloy steel may consist essentially of, essentiallyconsist of, or consist of: an amount of about 0.35 to 0.45 wt % ofcarbon (C), an amount of about 0.80 to 1.20 wt % of silicon (Si), anamount of about 0.20 to 0.50 wt % of manganese (Mn), an amount of about6.00 to 8.00 wt % of chromium (Cr), an amount of about 1.50 to 3.00 wt %of molybdenum (Mo), an amount of about 0.80 to 1.20 wt % of vanadium(V), an amount of about 0.05 to 0.10 wt % niobium (Nb), an amount ofabout 0.10 to 1.00 wt % of tungsten (W), and iron (Fe) constituting theremaining balance of the alloy steel, based on the total weight of thealloy steel.

Another exemplary embodiment of the present invention provides a moldincluding the alloy steel composition as described herein. By using thealloy steel of the present invention, a mold may have an improvedlifespan and extended life span.

Furthermore, the alloy steel particularly comprising molybdenum,tungsten, and/or niobium according to the long life alloy tool steel ofthe present invention may provide improved strength at high temperature,enhanced temper softening resistance, enhanced hot abrasion resistance,and improved toughness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates change in cavity of a mold according to the relatedart.

FIG. 2 shows breakage of a forging mold according to the related art.

FIG. 3 shows molybdenum carbide of an exemplary steel for an exemplarymold tool according to an exemplary embodiment of the present invention.

FIG. 4 is a graph illustrating an yield strength according to anexemplary content of molybdenum of an exemplary alloy steel for anexemplary mold tool according to an exemplary embodiment of the presentinvention.

FIG. 5 is a graph illustrating tensile strength according to anexemplary content of molybdenum of an exemplary alloy steel for anexemplary mold tool according to an exemplary embodiment of the presentinvention.

FIG. 6 is a graph illustrating hardness according to an exemplaryaustenizing temperature in an alloy steel composition according toComparative Example 1 which is the related art and when tungsten isadded to a mold tool steel according to Comparative Example 1 which isthe related art.

FIG. 7 shows tungsten carbide of an exemplary alloy steel for anexemplary mold tool according to an exemplary embodiment of the presentinvention.

FIG. 8 is a graph illustrating toughness according to an exemplarycontent of tungsten of an exemplary alloy steel for an exemplary moldtool according to an exemplary embodiment of the present invention.

FIG. 9 illustrates progression of cracks according to the size of largecrystal grains in the related art.

FIG. 10 illustrates progression of cracks according to the size of smallcrystal grains in the related art.

FIG. 11 is a graph illustrating stretching ratio according to anexemplary content of niobium of an exemplary alloy steel for anexemplary mold tool according to an exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The terminology used herein is for the purpose of describing particularexemplary embodiments only and is not intended to be limiting of theinvention. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises” and/or “comprising,” when used in this specification,specify the presence of stated features, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more other features, integers, steps, operations,elements, components, and/or groups thereof. As used herein, the term“and/or” includes any and all combinations of one or more of theassociated listed items.

Unless specifically stated or obvious from context, as used herein, theterm “about” is understood as within a range of normal tolerance in theart, for example within 2 standard deviations of the mean. “About” canbe understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%,0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear fromthe context, all numerical values provided herein are modified by theterm “about.”

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g. fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

Hereinafter, the exemplary embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings. Priorto the description, the terms or words used in the present specificationand the claims should not be construed as being limited as typical ordictionary meanings, and should be construed as meanings and conceptsconforming to the technical spirit of the present invention on the basisof the principle that an inventor can appropriately define concepts ofthe terms in order to describe his or her own invention in the best way.Accordingly, since the exemplary embodiments described in the presentspecification and the configurations illustrated in the drawings aregiven in an exemplary embodiment of the present invention and do notrepresent all of the technical spirit of the present invention, it is tobe understood that various equivalents and modified examples, which mayreplace these exemplary embodiments and configurations, are possible atthe time of filing the present application.

An aspect of the present invention relates to an alloy steel withimproved physical properties at high temperature and extended life span.In general, when parts of the vehicle are manufactured, the durabilityof the parts may be improved and the strength may be increased, so thatload imposed on a mold used to manufacture the parts may be increased.Furthermore, the load imposed on the mold may be increased due to anincrease in the number of cavities of the mold as shown in FIG. 1 inorder to improve the productivity of parts. For example, when the partsare usually manufactured using a forging press, a first process (buster)includes a process for distributing the volume, a second process(blocker) includes a process for rough-shaping, a third process(finisher) includes a process for dimension molding, a fourth process(piercing) includes a process for separating the inner flash, and afifth process (trimming) includes a process for separating the outerflash. Among the processes, the high load processes may be the first tothird processes. Due to the first to third processes as described above,the mold may be broken due to the high load as illustrated in FIG. 2.

In order to solve the problems as described above, a conventional steel(Comparative Example 3) in the related art may include a steel to whichan inexpensive alloy specialized for forging may be added. Theconventional steel may have reduced amounts of expensive alloycomponents, such as vanadium (V) and molybdenum (Mo), which may berequired to secure the hardness at an unnecessary high temperature. Inaddition, the conventional steel may improve strength and toughnessthrough the crystal grain refinement resulting from the addition ofniobium (Nb) in order to secure abrasion resistance and impact toughnessof the mold tool steel, and may secure hardness and abrasion resistanceat normal temperature due to the increase in hardenability by addingchromium (Cr) and adding boron (B).

TABLE 1 Component (wt %) C Si Mn Ni Cr Mo V Nb B Comparative 0.32 to0.80 to 0.50 or — 4.50 to 1.00 to 0.80 to — — Example 1 0.42 1.20 less5.50 1.50 1.20 Comparative 0.49 to 0.15 to 0.70 to 1.50 to 0.70 to 0.20to 0.10 to — — Example 2 0.54 0.35 1.00 1.80 1.20 0.40 0.15 Comparative0.35 to 0.70 to 0.50 or — 6.00 to 0.70 to 0.40 to 0.10 to 0.001 toExample 3 0.45 0.90 less 8.00 0.90 0.60 0.20 0.002

Table 1 shows components in Comparative Example 1 to 3 according to therelated art, and shows that iron (Fe) was included as a main componentand components added are shown as weight % (wt %) based on the totalweight of the entire mold tool steel.

In Comparative Example 3, the strength and hardness were in the samelevel as those in Comparative Example 2, when the same heat treatment asperformed, and the strength and hardness were increased by 20% comparedto Comparative Example 2. For example, about 14,000 strokes wereachieved by a pair of molds used to produce an R engine con rod, and themanufacturing costs of parts were reduced. Further, Comparative Examples1 to 3 were all in the same level in terms of impact toughness, but inthe forging evaluation actually applied, the life in ComparativeExamples 1 to 3 was in the same level as that of Comparative Example 2and the life was improved by about 5 times or greater compared toComparative Example 2. However, the conventional alloy steel compositionsuch as Comparative Example 3 may have a problem in that the strength,hardness, and abrasion resistance required for a mold applied to massproduction are not satisfied.

Accordingly, provided is an alloy steel for a mold too, which mayinclude an amount of about 0.35 to 0.45 wt % of carbon (C), an amount ofabout 0.80 to 1.20 wt % of silicon (Si), an amount of about 0.20 to 0.50wt % of manganese (Mn), an amount of about 6.00 to 8.00 wt % of chromium(Cr), an amount of about 1.50 to 3.00 wt % of molybdenum (Mo), an amountof about 0.80 to 1.20 wt % of vanadium (V), an amount of about 0.05 to0.10 wt % of niobium (Nb), an amount of about 0.10 to 1.00 wt % oftungsten (W), and iron (Fe) constituting the remaining balance of thealloy steel, and all the wt % are based on the total weight of the alloysteel.

The process for the alloy steel composition of the present invention mayinclude a forging process, and the maximum temperature of a mold for hotforging used at the highest temperature in the forging process may be ofabout 500° C. Accordingly, the content of alloy elements for improvingphysical properties at high temperature of about 500° C. may beoptimized. By increasing the content of molybdenum (Mo) and chromium(Cr) compared to the related art, the strength and temper softeningresistance at high temperature may be improved. Further, tungsten may beadded to improve hardness at a high temperature due to the precipitationof fine tungsten (W) carbide (carbide), and the strength and toughnessmay be simultaneously improved through crystal grain refinement byadding niobium (Nb).

Hereinafter, the effects due to the addition of each alloy element ofthe present invention will be specifically examined.

(1) Carbon (C)

The carbon (C) as used herein may be an important element for securingthe strength of an alloy steel, and may stabilize the residualaustenite. For the role, the content of carbon (C) may be preferablyabout 0.35 to 0.45 wt % based on the total weight of the alloy steel.

Here, when the content of carbon (C) is less than about 0.35 wt %, thestrength of the mold tool steel may not be sufficiently obtained, andthe reduction in strength may be incurred, and the like. In contrast,when the content of carbon (C) is greater than about 0.45 wt %,undissolved large carbide may remain, thereby leading to reduction instrength and durability, and the like.

(2) Silicon (Si)

The silicon (Si) as used herein may be a deoxidizer, and may suppressformation of pinholes in a mold tool steel. The silicon component may besolid-solubilized in a matrix to increase the strength of an alloy steelby the solid solution strengthening effect, and enhance the activity ofcarbon (C), and the like. For the role, the content of silicon (Si) maybe preferably about 0.80 to 1.20 wt % based on the total weight of thealloy steel.

When the content of silicon (Si) is less than about 0.80 wt %, oxidesmay remain in the alloy steel due to oxygen which may not besufficiently removed, as consequence, the strength of the mold toolsteel may be reduced, sufficient solid solution strengthening effectsmay not be obtained. When the content of silicon (Si) is greater thanabout 1.20 wt %, decarburization may be generated by an interpermeationreaction in the structure, such as a site competition reaction withcarbon (C) due to the excessive content of silicon (Si).

(3) Manganese (Mn)

The manganese (Mn) as used herein may improve the hardenability of amold tool steel, and enhance the strength of the mold tool steel, andthe like. Preferably, the content of manganese (Mn) may be about 0.20 to0.50 wt % based on the total weight of the alloy steel.

When the content of manganese (Mn) is less than about 0.2 wt %, aneffect of improving the hardenability of a mold tool steel may not besufficient. In contrast, when the content of manganese (Mn) is greaterthan about 0.50 wt %, the processability and the life may deteriorate.

(4) Chromium (Cr)

The chromium (Cr) as used herein may improve the hardenability of a moldtool steel, imparts curability, and refine structure of the alloy steel.Further, the chromium (Cr) may enhance the strength at high temperature,and enhance the temper softening resistance. Preferably, the content ofchromium (Cr) may about 6.00 to 8.00 wt % based on the total weight ofthe alloy steel.

When the content of chromium (Cr) is less than about 6.00 wt %, thehardenability and curability may be limited, and sufficient structurerefinement and spheroidization may not be obtained. When the content ofchromium (Cr) is greater than about 8.00 wt %, the effect caused by anincrease in content may not be sufficient, and thus, an increase in themanufacturing costs may be incurred.

(5) Vanadium (V)

The vanadium (V) as used herein may form a precipitate such as carbide,and strengthen the matrix structure thereby enhancing strength andabrasion resistance through the precipitation strengthening effect, andreducing the activity of carbon. Further, the strength of the alloysteel may be increased at the same cooling rate. Preferably, the contentof vanadium (V) may be about 0.80 wt % to 1.20 wt % based on the totalweight of the alloy steel.

Here, when the content of vanadium (V) is less than 0.80 wt % or greaterthan about 1.20 wt %, toughness and hardness of an alloy steel, and thelike may be reduced.

(6) Molybdenum (Mo)

The molybdenum (Mo) as used herein may enhance the strength at hightemperature by precipitation of molybdenum (Mo) carbide and increase thetemper softening resistance. FIG. 3 shows an enlarged photographillustrating molybdenum carbide of an exemplary alloy steel according toan exemplary embodiment of the present invention. As indicated by thearrow inside FIG. 3, molybdenum (Mo) carbide may be precipitated.Preferably, the content of molybdenum (Mo) may be about 1.50 to 3.0 wt %based on the total weight of the alloy steel.

When the content of molybdenum (Mo) is less than about 1.50 wt %, asufficient strength may not be secured. When the content of molybdenum(Mo) is greater than about 3.0 wt %, the effects of yield strength andtensile strength may be reduced, and the effects due to an increase incontent may not be sufficient, thereby incurring an increase in themanufacturing costs.

When this is further reviewed, FIG. 4 is a graph illustrating the yieldstrength according to an exemplary content of molybdenum of an exemplaryalloy steel according to an exemplary embodiment of the presentinvention. In the graph of FIG. 4, the horizontal axis indicates thecontent of molybdenum and the unit is wt %, and the vertical axisindicates the yield strength and the unit is MPa. As illustrated in FIG.4, strength may be substantially increased when the content ofmolybdenum is about 1.0 to 1.5 wt %. However, the increase rate instrength is rapidly decreased when the content of molybdenum is about3.0 to 3.5 wt %.

Further, FIG. 5 is a graph illustrating the tensile strength accordingto an exemplary content of molybdenum of an exemplary alloy steelaccording to an exemplary embodiment of the present invention. In thegraph of FIG. 5, the horizontal axis indicates the content of molybdenumand the unit is wt %, and the vertical axis indicates the tensilestrength and the unit is MPa. As illustrated in FIG. 5, strength may besubstantially increased when the content of molybdenum is about 1.0 to1.5 wt %. However, the increase rate in strength may be rapidlydecreased when the content of molybdenum is about 3.0 to 3.5 wt %.

TABLE 2 Content of Mo (wt %) Tensile strength (MPa) 1.0 520 1.5 610 2.0670 2.5 700 3.0 700 3.5 710

In Table 2, the tensile strength was measured by changing only thecontent of molybdenum while the components of the present invention werethe same as each other. As shown in Table 2, when molybdenum was addedin an amount of 1.0 to 1.5 wt % based on the total weight of the alloysteel, the increase rate was 90 MPa, which was a rapidly increasedvalue. When molybdenum was added in an amount of about 3.0 to 3.5 wt %based on the total weight of the alloy steel, the increase rate wasabout 10 MPa, which is a small increasedvalue.

(7) Tungsten (W)

The tungsten as used herein may enhance hardness, abrasion resistance,and toughness at high temperature because tungsten carbide may beprecipitated. Preferably, the content of tungsten may be about 0.1 wt %to about 1.0 wt % based on the total weight of the alloy steel.

When the content of tungsten is less than about 0.10 wt %, hardness maynot be sufficiently improved because tungsten carbide is notsufficiently precipitated. When the content of tungsten is greater thanabout 1.00 wt %, impact toughness may be reduced by precipitation ofcoarse tungsten carbide (carbide).

FIG. 6 is a graph illustrating the hardness according to the austenizingtemperature in an alloy steel according to Comparative Example 1. InFIG. 6, the horizontal axis indicates the austenizing temperature andthe unit is ° C., and the vertical axis indicates the hardness and theunit corresponds to HRC. In FIG. 6, when tungsten was added toComparative Example 1, the hardness according to the austenizingtemperature was increased as compared to that of Comparative Example 1where tungsten was not added. FIG. 7 is an enlarged photographillustrating tungsten carbide of an exemplary alloy steel according toan exemplary embodiment of the present invention. When the temperatureis equal to or greater than the austenizing temperature, tungstencarbide may be precipitated, and as indicated by the arrow in FIG. 7,the precipitated tungsten carbide may be confirmed. Accordingly, thetungsten carbide in the alloy steel of the present invention mayincrease the hot abrasion resistance.

However, when the content of tungsten is greater than about 1.0 wt %,impact toughness is lowered by precipitation of coarse tungsten carbide.FIG. 8 is a graph illustrating the toughness according to an exemplarycontent of tungsten of an exemplary alloy steel according to anexemplary embodiment of the present invention. In FIG. 8, the horizontalaxis indicates the content of tungsten and the unit is wt %, and thevertical axis indicates the toughness and the unit corresponds to J. Asillustrated in FIG. 8, when the content of tungsten was about 0.1 wt %to 1.0 wt %, tungsten carbide was stabilized. When the content wasgreater than about 1.0 wt %, that carbide (carbide) may be coarsened bytungsten, and impact toughness may be reduced.

(8) Niobium (Nb)

When niobium is added, the toughness may be prevented from being reduceddue to the refinement of crystal grains, and the corrosion fatigue lifeof the material may be enhanced. This may be proved by the followingEquation 1.

σ₀=σ_(i) +K′d ^(−1/2)  [Equation 1]

-   -   σ=Yield stress toughness    -   σ=Dislocation motion obstruction frictional coefficient    -   K′=Barrier integration constant of dislocation    -   d=Diameter of crystal grains

According to Equation 1 (Hall-Petch equation), as the diameter ofcrystal grains is decreased, strength and toughness are increased. FIG.9 is a schematic view illustrating the progression of cracks accordingto the size of large crystal grains, and FIG. 10 is a schematic viewillustrating the progression of cracks according to the size of smallcrystal grains. When external force acts from the left side to the rightside in FIGS. 9 and 10, the arrow indicates that cracks progress.Accordingly, the refiner the crystal grains are, the more the number ofsteps of progressing cracks is, and the corrosion fatigue life may beincreased because it is difficult for cracks to progress.

TABLE 3 Content of Nb (wt %) Strength (MPa) Stretching ratio (%) 0.021,793 10.3 0.04 1,788 11.7 0.06 1,802 14.9 0.09 1,814 14.8 0.12 1,83115.0

Table 3 shows an effect of enhancing the strength and the stretchingratio according to the amount of niobium component added in thecomponents of the present invention. In Table 3, there may be adifference in stretching ratio according to the amount of Nb added.

When niobium was added in an amount of 0.02 wt %, the strength was 1,793MPa and the stretching ratio was 10.3%. Further, when niobium was addedin an amount of about 0.04 wt %, the strength was 1,788 MPa and thestretching ratio was 11.7%. Furthermore, when niobium was added in anamount of 0.06 wt %, the strength was 1,802 MPa and the stretching ratiowas 14.9%. In addition, when niobium was added in an amount of 0.09 wt%, the strength was 1,814 MPa and the stretching ratio was 14.8%.Finally, when niobium was added in an amount of 0.12 wt %, the strengthwas 1,831 MPa and the stretching ratio was 15.0%. FIG. 11 is a graphillustrating the stretching ratio according to an exemplary content ofniobium of an exemplary alloy steel according to an exemplary embodimentof the present invention. In FIG. 11, the horizontal axis of the graphindicates the content of niobium and the unit is wt %, and the verticalaxis indicates the stretching ratio and the unit is %. As illustrated inFIG. 11, the stretching ratio may be rapidly increased from the pointwhere the content of niobium is about 0.5 wt % based on the total weightof the alloy steel.

Accordingly, the content of niobium of the present invention preferablymay be about 0.05 to 0.10 wt % based on the total weight of the alloysteel. When the content of niobium is less than about 0.05 wt %, thelife may be shortened, and when the content of niobium is greater thanabout 0.10 wt %, the effect of enhancing the stretching ratio may beminimal, and thus the manufacturing costs may be increased.

In another aspect, the present invention relates to a mold including thealloy steel as described herein.

The mold may have substantially improved physical properties at hightemperature, and may have an effect in that the life of the mold, whichmay be enhanced by about 40%. For example, 20,000 strokes may be usedwhen the mold of the present invention is applied to the actual forgingprocess whereas only 14,000 strokes may be used in the related art.

according to various exemplary embodiments, by adding molybdenum,tungsten, and niobium, strength at high temperature and temper softeningresistance of the alloy steel may be increased, hot abrasion resistancethereof may be enhanced, and strength and toughness thereof also may beincreased. Furthermore, by manufacturing a mold including the alloysteel of the present invention, the life of the mold may be increased.

As described above, the present invention has been described in relationto exemplary embodiments of the present invention, but the exemplaryembodiments are only illustration and the present invention is notlimited thereto. Exemplary embodiments described may be changed ormodified by those skilled in the art to which the present inventionpertains without departing from the scope of the present invention, andvarious alterations and modifications are possible within the technicalspirit of the present invention and the equivalent scope of the claimswhich will be described below.

What is claimed is:
 1. An alloy steel for a mold tool, comprising:amount of about 0.35 to 0.45 wt % of carbon (C), amount of about 0.80 to1.20 wt % of silicon (Si), amount of about 0.20 to 0.50 wt % ofmanganese (Mn), amount of about 6.00 to 8.00 wt % of chromium (Cr),amount of about 1.50 to 3.00 wt % of molybdenum (Mo), amount of about0.80 to 1.20 wt % of vanadium (V), and iron (Fe) constituting theremaining balance of the alloy steel, all the wt % based on the totalweight of the alloy steel.
 2. The alloy steel of claim 1, furthercomprising niobium (Nb).
 3. The alloy steel of claim 2, wherein acontent of the niobium (Nb) is about 0.05 to 0.10 wt % based on thetotal weight of the alloy steel.
 4. The alloy steel of claim 1, furthercomprising tungsten (W).
 5. The alloy steel of claim 4, wherein acontent of the tungsten (W) is amount of about 0.10 to 1.00 wt % basedon the total weight of the alloy steel.
 6. The alloy steel of claim 1,further comprising niobium (Nb) and tungsten (W).
 7. The alloy steel ofclaim 6, wherein a content of the niobium (Nb) is amount of about 0.05to 0.10 wt % and a content of the tungsten (W) is amount of about 0.10to 1.00 wt % based on the total weight of the alloy steel.
 8. The alloysteel of claim 1, consisting essentially of: amount of about 0.35 to0.45 wt % of carbon (C), amount of about 0.80 to 1.20 wt % of silicon(Si), amount of about 0.20 to 0.50 wt % of manganese (Mn), amount ofabout 6.00 to 8.00 wt % of chromium (Cr), amount of about 1.50 to 3.00wt % of molybdenum (Mo), amount of about 0.80 to 1.20 wt % of vanadium(V), and iron (Fe) constituting the remaining balance of the alloysteel, all the wt % based on the total weight of the alloy steel.
 9. Thealloy steel of claim 1, consisting essentially of: amount of about 0.35to 0.45 wt % of carbon (C), amount of about 0.80 to 1.20 wt % of silicon(Si), amount of about 0.20 to 0.50 wt % of manganese (Mn), amount ofabout 6.00 to 8.00 wt % of chromium (Cr), amount of about 1.50 to 3.00wt % of molybdenum (Mo), amount of about 0.80 to 1.20 wt % of vanadium(V), amount of about 0.05 to 0.10 wt % of niobium (Nb), and iron (Fe)constituting the remaining balance of the alloy steel, all the wt %based on the total weight of the alloy steel.
 10. The alloy steel ofclaim 1, consisting essentially of: amount of about 0.35 to 0.45 wt % ofcarbon (C), amount of about 0.80 to 1.20 wt % of silicon (Si), amount ofabout 0.20 to 0.50 wt % of manganese (Mn), amount of about 6.00 to 8.00wt % of chromium (Cr), amount of about 1.50 to 3.00 wt % of molybdenum(Mo), amount of about 0.80 to 1.20 wt % of vanadium (V), amount of about0.10 to 1.00 wt % of tungsten (W), and iron (Fe) constituting theremaining balance of the alloy steel, all the wt % based on the totalweight of the alloy steel.
 11. The alloy steel of claim 1, consistingessentially of: amount of about 0.35 to 0.45 wt % of carbon (C), amountof about 0.80 to 1.20 wt % of silicon (Si), amount of about 0.20 to 0.50wt % of manganese (Mn), amount of about 6.00 to 8.00 wt % of chromium(Cr), amount of about 1.50 to 3.00 wt % of molybdenum (Mo), amount ofabout 0.80 to 1.20 wt % of vanadium (V), amount of about 0.05 to 0.10 wt% of niobium (Nb), amount of about 0.10 to 1.00 wt % of tungsten (W),and iron (Fe) constituting the remaining balance of the alloy steel, allthe wt % based on the total weight of the alloy steel.
 12. A moldcomprising a steel of claim 1.